New USDA Plant Hardiness Map Reflects Warming

The new USDA plant hardiness zone map, unveiled in January.
As may or may not surprise you, the colorful United States map of planting zones is being updated for a warmer 21st Century.

Hardiness zone maps for the United States were developed during the 1920s and 1930s to help farmers plan for their regions' growing seasons. The first USDA Map was published in 1960 (USDA Miscellaneous Publication No. 814 "Plant Hardiness Zone Map.") and was developed by Henry Skinner while he was the Director of the U.S. National Arboretum. The map showed ten broad hardiness zones based on 10 degree F. gradients:

By 1990, the USDA version of hardiness zone maps became the primary source for zone identification. That same year, H. Marc Cathey completed a major overhaul of the USDA map using temperature data from 1974 to 1986. One new zone was added to coincide with adding Mexico and Canada to the map and the prior 10-degree gradients were broken down into 5- degree a and b zones. One of the primary reasons given for the update was that, "We have been losing from our landscapes plants that apparently survived the 1940's to the 1960's. Many of the hardiness zone classifications of plants are no longer considered valid. In North America, the ranges of temperature and moisture for the past decade were wider than those recorded for the 1940s through the 1960s."

That statement—made more than 20 years ago—is interesting for two reasons: One, it presages the current debate over the proposed update to the old 1990 USDA map—and two, for its implication that the hardiness zones need to reflect recent global warming.

The latest update to the hardiness zone map unveiled in January, is the first since that 1990 overhaul and includes two new zones added in hotter climates, bringing the number of mapped zones up to 13.

The new map is located on the USDA website and was designed for the Web, allowing people to enter their ZIP code and see their zone down to half-mile segments. That eliminates the guesswork for the nation's 82 million gardeners.

Catherine Woteki, agriculture undersecretary for research, education and economics, said the changes to the map don't indicate permanent climate change. No section of the country changed more than half a zone, according to Woteki. She says scientists used the latest complete data, adding they performed an analysis to see if incomplete data from the past five years would require alterations.

"We saw it would not change the map," she told USA Today.

The USDA website notes: "Climate changes are usually based on trends in overall average temperatures recorded over 50-100 years. Because the (new map) represents 30-year averages of what are essentially extreme weather events (the coldest temperature of the year), changes in zones are not reliable evidence of whether there has been global warming."

Needless to say, something's amiss. Check out this cheat-sheet of changes:
  • Much of the Northeast is half-zone warmer. For example, Pennsylvania, which was about equally divided between zone 5 and zone 6 in the 1990 map is now about 70% zone 6 and 30% zone 5.
  • Nebraska was mostly zone 4 and is now almost entirely zone 5.
  • Ohio was mostly zone 5 and now is mostly zone 6.
  • South Florida and Southern California have new hotter areas around cities, but California also has some colder areas in the mountains.
Check out this news report:

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Nicole Caldwell

Nicole Caldwell is a self-taught environmentalist, green-living savant and sustainability educator with more than a decade of professional writing experience. She is also the co-founder of Better Farm and president of betterArts. Nicole’s work has been featured in Mother Earth News, Reader’s Digest, Time Out New York, and many other publications. Her first book, Better: The Everyday Art of Sustainable Living, is due out this July through New Society Publishers.